1. Field of the Invention
The invention relates to tunable external cavity lasers and more particularly, to methods and apparatus for tuning external cavity lasers.
2. Description of Related Art
The basic principles of the operation of the tunable laser utilizing a variable length external cavity in conjunction with a diffraction grating and a rotatable mirror are set forth in the publication, "Spectrally Narrow Pulse Dye Laser Without Beam Expander," by Michael G. Littman and Harold J. Metcalf, Applied Optics, vol. 17, No. 14, pages 2224-2227, Jul. 15, 1978. Although the article describes a system which uses a dye laser, the diode laser is easily substituted. The system utilizes a diffraction grating which is filled with an incident collimated laser beam by using the grating at a grazing angle. The diffracted beam at the angle normal to the mirror is reflected back onto the grating and from there it is diffracted in a direction opposite the original collimated beam. The first order of diffraction of the grating is incident on the mirror, which reflects it back onto the grating, where the first order of diffraction passes back into the gain medium, where it serves to determine the operating wavelength of the laser. The output of the system is the zero-order reflection from the grating at grazing incidence. Motion of the mirror with respect to the grating allows the system to be tuned to a desired output wavelength.
The above mentioned design is susceptible to discontinuities in the output spectrum. These discontinuities are caused by mode hopping which is a change in the integral number of wavelengths in the cavity over the tuning range. To overcome mode hopping U.S. Pat. No. 5,319,668 teaches a pivot point for the reflective element, e.g. mirror or dihedral reflector, which provides for simultaneous rotary and linear motion with respect to the grating and thus theoretically overcomes the problem of mode hopping. The pivot point is selected so as to provide an internal cavity length which is exactly an integral number of half wavelengths at three different wavelengths and an exceptionally close (within 1/1000 of one wavelength) match at all other wavelengths within the tuning range. The pivot point calculation takes into account the effect of the dispersion of the gain medium and other optical elements in the system on the cavity length.
While this arrangement represents a significant improvement compared to previous external cavity diode lasers by providing a pivot capable of tuning over a wide range of wavelengths, it still has a number of drawbacks. Chief among these is the expense of the precision bearings needed for pivoting the reflective element about the pivot point. The bearings are extremely expensive because they need to be stable on a nanometer scale over their range of motion. The fineness of machining needed to accomplish these extreme tolerances makes the price of the bearings extremely high.
What is needed is a design for tuning external cavity semiconductor diode lasers (ECDL) which does not require the expense and precision of prior art bearing designs and is not susceptible to mode hopping. What is needed is a design that allows for a pivot point for the reflector which provides for both rotary and lineal translation of the reflector with respect to the grating, without expensive bearings. What is needed is a pivot design that will maintaining a constant number of half-wavelengths within the cavity across the entire tuning range without the precision of prior art designs. What is needed is a design that provides for a broad tunable range of output wavelengths.